The present invention relates to mineral separation, and more particularly, to the separation of paramagnetic minerals from nonmagnetic minerals using high gradient magnetic fields.
High gradient magnetic separators have been particularly useful in the kaolin clay industry. Kaolin clay is a nonmagnetic material that, when mined, contains traced quantities of paramagnetic impurities such as titanium dioxide (TiO.sub.2) and iron oxide (FeO.sub.2). The magnetic impurities are removed from the kaolin clay by mixing the clay with water to form a slurry. The slurry is then pumped through a separation container that is subjected to an intense magnetic field and packed with stainless steel wool. The intense magnetic field causes high magnetic field gradients to form around the fibers of the steel wool causing the paramagnetic impurities to be attracted to the steel wool. Over time, the stainless steel wool becomes saturated with a buildup of the paramagnetic impurities, reducing the effectiveness of the magnetic separator. Past developments in magnetic separators have focused on improving the efficiency of generating the magnetic field in the separation container and removing the paramagnetic impurity buildup.
Initially, conventional copper coils were used to generate the magnetic field for mineral separation. Unfortunately, copper coils have a finite resistance and, to maintain a high gradient magnetic field, must be continuously supplied with a large quantities of electrical power. More particularly, in a magnetic solenoid, typical of those used in a magnetic separator, the ampere-turns (NI) of the solenoid are proportional to the desired flux density (B) times the magnetic reluctance () of the solenoid. When an iron casing is used in the regions outside of the solenoid and the separation container and the magnetic field is below about 2 Tesla, most of the magnetic reluctance in the solenoid is caused by an air gap formed by the separation container. Also, magnetic reluctance is proportional to the size of the air gap (G) or, more specifically; the distance between the "pole" faces on the iron casing. An increase in the air gap results in an increase in the necessary current (I) and/or an increase in the number of coil turns (N) if a particular flux density is to be achieved.
The copper coil's resistance is proportional to the diameter of the coil times the number of turns (N). Since power (P) equals current (I) squared times resistance (R), the power required for a conventional copper coil is proportional to the desired magnetic flux density (B), times the gap (G), times the diameter (D), times the current (I). Thus, an increase in diameter in a conventional copper coil results in a directly proportional increase in power, and an advantageous volume increase proportional to the diameter squared. However, an increase in gap (G) results in a directly proportional power increase and a less advantageous, directly proportional volume increase. For these reasons, increasing the air gap in a copper-coil based separator required a large proportional increase in electrical power consumption and, for cost and efficiency reasons, generally was not considered a viable option.
These factors induced magnetic separators using conventional copper coils to be designed to use relatively flat, pancake-like, separation containers. Thus, existing copper coil based magnetic separators typically use a separation container having a diameter from about 213 centimeters (84 inches) to about 305 centimeters (120 inches) and a height of about 50 centimeters (20 inches). The 305 centimeter (120 inches) separation container diameter was considered an improvement over previous containers having a diameter of 84 inches. However, the increased diameter increased the separator's weight from about 226,800 kilograms (250 tons) to about 635,000 kilograms (700 tons). Magnetic saturation of the iron in the casing limited the magnetic field strength within the separation container to about 2 Tesla.
Efforts to reduce the power required to generate the magnetic field in the separation container resulted in the development of electromagnets using superconducting coils. These efforts were generally directed toward "drop in" replacements for existing copper coils, and thus most superconducting coils used in magnetic separators today have geometries substantially identical to the geometries of the copper coils they have replaced. A main advantage of using a superconducting coil in a magnetic separator is that a superconductor has virtually no resistance. Accordingly, once a current is induced in a superconducting coil, no or little additional electrical power is need to maintain the current.
When the stainless steel wool becomes saturated with a buildup of magnetic impurities, the current in the superconducting coil is periodically ramped down to permit flushing of the magnetic impurities from the stainless steel wool. Such flushing is achieved by flowing high pressure water and air through the separation container (in alternating forward and reverse directions). Following the flushing, the current in the superconducting coil is then ramped up again and the separation repeated. Ramping times can be, for example, in the order of 60 seconds.
Another approach for removing the buildup of magnetic impurities on the stainless steel wool involves periodically moving the entire separation container out of the magnetic field generated by the superconducting coil to permit flushing in the manner described above. In accordance with this approach, current within the superconducting coil remains constant, thus eliminating the need for ramping. In a sense, this approach represents a mechanical solution to an electromagnetic problem, i.e., removing the magnetic field from the separation container and the stainless steel wool prior to flushing. Unfortunately, however, the moving of the separation container necessitates the opening and closing of numerous valves and the capacity of such a device is limited by the necessity of moving a large canister in and out of the magnetic field.
Other magnetic separators exist that are generally used for experimental purposes. These separators typically have a long tubular separation container. However, these long tube separators have provided only modest performance increases and have limited capacity.
Accordingly, there exists a need for a magnetic separator having good power efficiency while providing superior throughput performance. The present invention satisfies this need.